1. Field of the Invention
The present invention relates to polymer supported transition metal catalysts and more particularly to the preparation of these catalysts by photochemical addition of the transition metal compound to the polymer support and to the use of the catalysts in chemical reactions under mild conditions.
2. Description of the Prior Art
The relative merits of homogeneous and heterogeneous catalysts are well known. Homogeneous catalysts have better defined active sites, usually have all of the metal available for catalysis, and offer steric and electronic environments of the metal atom that can, at least in principle, be varied at will. The major disadvantage of homogeneous catalysts is the need to separate them from reaction products without loss of their valuable metal content. This step can be both complex and expensive. Other disadvantages are that these catalysts are relatively easily deactivated through aggregation or by poisonous by-products or at extreme temperatures. Also, corrosion of reactors by metal complexes is common.
In the last few years many advances have been made in transition metal homogeneous catalysis. Efficient transition metal catalysts are known for a variety of useful reactions, such as the reduction of unsaturated organic molecules, and the hydroformylation, the polymerization, and the oxidation of olefins. Homogeneous catalysts are generally more selective, and function under less rigorous conditions than heterogeneous catalysts. Many homogeneous transition metal complexes have their effectiveness reduced by the fact that they undergo undesirable side reactions, such as dimerization. To eliminate this problem, to enhance catalyst selectivity, and to facilitate catalyst-product separation a few experimenters have attached catalytic metal complexes to solid supports, particularly to polymeric supports.
Most workers have used complexes in which phosphine groups are used to link the metal to the solid support. Two types of polymer support, modified polystyrene and silica, have been studied. With polystyrene the form of the polymer can be changed by changing the amount of cross-linking, a feature that appears to have important consequences on the type of catalyst produced. This is the type of support that has been most widely used.
Several useful catalysts have been created from solid-supported metal complexes such as the catalyzed hydrogenation of olefins with polymerically supported palladium (II) and platinum (II) complexes. Catalytic reduction of olefins was also achieved by using polymer-supported tris(triphenylphosphine) chlororhodium (I). Autoclave hydroformylation of cyclohexene was performed by polymer-bound dicobalthexacarbonyl, and ethyl propionate was polymerized into aromatic compounds using Ni(CO).sub.2 (PPh.sub.3).sub.2 as a polymer-attached species. Titanocene dichloride was considerably enhanced in its catalytic ability by attachment to polystyrene polymers. Dimerization of the titanocene was effectively prohibited by isolation of the reactive sites on the polymeric support.
However, all of these systems rely on thermal activation of the catalysts and the olefin reactions are usually conducted at high temperature and pressure. Furthermore, many of the metal-polymer complexes are unstable, the metal gradually detaching from the polymer and entering the solution, making it commercially unfeasible to utilize expensive metals such as rhodium as a catalyst component.
Homogeneous phase reactions under photochemical conditions also have been observed. The disadvantages are the same as experienced with thermal homogeneous phase catalysis and additionally, aggregation or chelation effects are observed forming catalytically inactive compounds and the aggregates sometimes absorb in the same region at higher intensity terminating the reaction due to the internal filter effect.